Current practice in the nuclear industry requires fabrication of relatively small (10 millimeter diameter) cylindrical nuclear fuel pellets. These nuclear fuel pellets are typically assembled end-to-end within an elongated cylindrical tube, often termed cladding. The strict requirements for safety and quality in the nuclear industry requires the cylindrical surface of fuel pellets to be optically inspected prior to assembly into the cladding to make fuel pins. Optical inspection is needed to assure that the cylindrical surfaces are not irregular, pitted, or otherwise defective since such defects can cause localized overheating which may damage or cause failure of the fuel pin cladding.
Nuclear fuels are typically radioactive and can also be chemically toxic to human beings. Toxicity is a particular problem with plutonium fuels and fuels containing mixed oxides of plutonium and uranium. Such toxic or radioactive pellets cannot be handled by humans and therefore must be remotely inspected.
The prior art includes U.S. Pat. No. 4,377,238 to Wilks et al. The Wilks patent discloses a method and apparatus using optics for inspecting nuclear fuel pellets for surface defects. The Wilks apparatus utilizes a laser beam which is scanned across the surface of a rotating pellet using a rotating polygonal mirror. Light images reflected from the surface of the rotating fuel pellet are detected by a photocell. Image signals for small individual image points are compiled on computer and analyzed to produce a quality index used to discriminate acceptable from not acceptable fuel pellets. Operational difficulties were experienced with inventions constructed according to the Wilks et al patent because of the complexity and small tolerance for error associated with rapidly scanning a laser beam across a rapidly revolving fuel pellet.
The prior art also includes U.S. Pat. No. 4,082,188 to Grimmel et al, entitled Apparatus For Color Recognition And Defect Detection of Objects Such As Capsules. The Grimmel patent discloses a system for automatically inspecting, analyzing and eliminating defective pharmaceutical capsules based on the optical characteristics of the exterior surfaces of the capsules. Focused light is reflected off of the capsules and gathered by two lens subsystems specially adapted for color recognition and structural defect recognition. Both subsystems employ photodetectors such as photomultipliers to transform the optical beams into analog electronic signals. The analog signals are converted into digital signals and a computer is used to compile and analyze the information. Instructions can then be made to reject defective capsules.